Metal nanoparticles such as nanospheres(1), nanorods(2, 3), nanocubes(4), nanoplates(5, 6), nanotetrapods(7), and nanoprisms(8) are attracting significant attention because of their size-dependent optical, magnetic, electronic and catalytic properties.(9-14) As but one example, gold nanoparticles can exhibit intense photoluminescence, a phenomenon which is expected to find wide scientific and practical use.(9)
The development of simple and versatile methods for the preparation of nanoparticles in a size or shape-selected and -controlled manner is an important and challenging task.(1-8, 15, 16) In addition, utilization of non-toxic chemicals, environmentally benign solvents, and renewable materials are emerging issues that merit important consideration in a synthetic strategy.(17)
Presently, the preparation of metal nanoparticles in solution most commonly involves chemical reduction of metal ions. In organic solvents, surfactant-stabilized reverse micellar (“water-in-oil”) systems have been used as “nanoreactors” for the chemical reduction of metal ions.(16, 18-20) In aqueous solutions(17, 21-27), metal nanoparticles have been typically produced from chemical reduction of metal ions by reducing agents dissolved in water; such reduction takes place in the presence of water-soluble polymers or surfactants and with the aid of externally supplied energy such as heat(17), photo-irradiation,(21) or ultrasound-irradiation(22). Such methods allow for some degree of control over the size and concentration of the dispersed particles.(21, 22, 26, 27)
However, concerns and problems remain. Present methods use organic solvents, produce byproducts due to the reducing agent, involve multiple steps, or require high concentration of protective agent to attain colloidal stability of the nanoparticles.(23) In an aim to address some of these problems, single-step synthesis of gold nanoparticles in aqueous solutions has been reported using poly(ethylene oxide)(24), diamine terminated poly(ethylene oxide)(25), amine-functionalized third-generation poly(propyleneimine) dendrimers(26) or α-biotinyl-PEG-block-[poly(2-(N,N-dimethylamino)ethyl methacrylate)](27). However, these recently-reported single-step methods may require utilization of “exotic” polymers, high temperatures(25-27) and high concentrations of protective agent.
Metal nanoparticle synthesis has been achieved by the use of a poly(ethylene oxide)-poly(propylene oxide) (PEO-PPO) type amphiphilic block copolymer(28, 29) In particular, silver nanoparticles are synthesized by mixing of [Ag(NH3)2]+ aqueous solution with PEO-PPO-PEO block copolymer (Pluronic P123) ethanol solution under ambient light at room temperature.(28) Gold nanowires and nanosheets are synthesized by UV irradiation photoreduction and thermal reduction processes in bulk copolymer (Pluronic P123).(29) However, these methods require the cumbersome use and removal of a cosolvent (tetrahydrofuran), the formation of a polymer film rather than a solution, and the application of an external energy source. Furthermore, no methods for the control particle formation, particle size and shape are disclosed.